As stored program-controlled switching systems have evolved, a wide variety of useful features have been developed to extend the communication capabilities such systems provide. Call waiting, a feature that uses a beep tone to inform a customer already engaged in an ongoing telephone conversation that a second call is coming in, is considered a convenience in many circumstances, allowing, for example, the interruption of an extended but relatively unimportant call to take an emergency, or long-awaited, priority call. However, there are several problems with the call waiting feature. Many customers would prefer not to have their telephone conversations interrupted, particularly by frequent routine or unwanted calls; customers with call waiting tend to interrupt a call to find out who is making a second call. If a customer with call waiting chooses to leave a second call unanswered, the second caller hears ringing and is unaware that the called party is busy on the telephone. The second caller may unnecessarily defer a subsequent call attempt based on the mistaken impression that the called party is not at home. Further, a call waiting interruption of a computer terminal data communication session is particularly undesirable. A customer may deactivate call waiting for such data sessions, but undesirably loses the benefit of call waiting interruption for important incoming calls for the duration of each, typically lengthy, data session.
In view of the foregoing, a recognized problem in the art is the failure of existing call waiting features to inform calling parties of the true call state at the called station so that unnecessary call interruptions and call attempt deferrals are minimized.